There are a number of systems for illuminating areas or objects for visual observation. The simplest system, i.e., critical illumination in microscopy, or searchlight optics on a larger scale, merely produces an image of its primary light source on the target area or object plane. If the primary light source does not have a uniform brightness, neither does its image, and consequently, the illumination is non-uniform, which is undesirable for many applications.
In order to obtain both well-defined and uniform illumination, two more complex systems have been used. One of these is a Kohler illumination system in which a non-uniform light source is imaged on a second lens by a first lensed of an image of the first lens formed of the second lens on the object plane. When used with a microscope, however, the Kohler system has a long fixed length, e.g., of about 12 cm. Due to its length, the Kohler system provides an obstruction to movements of the surgeon using the microscope in performing eye and other surgery.
The other system is a projection condenser system, which is similar to the Kohler illumination system and used with larger devices. As with the Kohler system, the distance between the primary light source and the object plane is fixed and is not practical, e.g., for medical microscopes.